A cross section of a conventional permanent magnet DC motor is shown in FIG. 7. Such motors uses a cylindrical shell 1. Permanent magnets 2 are fixed to an inner surface of the cylindrical shell 1 and are located about the periphery of a rotor 3. However, a motor with such a round structure is easy to roll about when being transported and the installation is inconvenience requiring an additional positioning device.
A motor having a square cross section developed in recent years is shown in FIG. 8. This motor uses a square shell 1′ having four side walls. Permanent magnets 2′ are fixed to the inner surface of the four side walls. Although such a motor with a square shell solves the above-mentioned shortcomings of conventional cylindrical motor, it has the following problems: 1) the shell has a low space utilization rate, and the space of the four corners are not used; 2) the diameter of rotor 3′ is equal to the width of shell 1′ minus the thickness of the side walls, the thickness of the permanent magnets 2′ and the width of air gap between magnets 2′ and rotor 3′. Attempts to reduce the size of the motor are restricted but the diameter of the rotor and the thickness of the magnets and the shell. As a result, the motor has low power density in a predetermined volume.